Apple has provided a concise rundown of how to update to Mountain Lion on day one here. The system requirements are the same for CustoMac Builds. To proceed with a retail installation, you absolutely need the following.

Intel Core 2 or above, 64 bit CPU

A computer running the latest version of Mac OS X Lion (10.7.x) or Snow Leopard (10.6.6+)

However, this time around there is a big change, as Mountain Lion is a 64-bit only Operating System. This means that there is no support for 32-bit only CPUs or 32-bit kexts. You will not be able to use the "arch=i386" boot flag to access 32-bit kernel and kexts mode. Some legacy 3rd party audio and wifi drivers may not work, and many older graphics drivers have been eliminated. If you want to check that you're currently using 64-bit kexts, this information can be viewed in System Profiler. Click on the Apple > About this Mac > More Info > System Info, click on Software. If the 64-bit Kernel and Extensions says Yes, you are clear to update.

Recommended Best Practices

When performing any full operating system upgrade, unexpected things can occur. To avoid data loss, these best practices should be adhered to. This upgrade should be a breeze, but if things do go wrong, you can always go back to your working Lion installation.